#include <bits/stdc++.h>
#define x first
#define y second
using namespace std;
using ll=long long;
using pii=pair<int,int>;
using vi=vector<int>;
using vl=vector<ll>;
#define pb push_back
#define all(a) begin(a),end(a)

const int N=55,MOD=1e9+7;
const char en='\n';
const ll LLINF=1ll<<60;

int n, m, cnt[2];
char c[N][N];
int idx[N][N];
vector <vi> v[2];

int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

bool ok (int x, int y) {
	return x >= 0 && x < n && y >= 0 && y < m && c[x][y] != '#';
}

void build_graph () {
	v[0].resize(cnt[0]);
	v[1].resize(cnt[1]);
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			if (c[i][j] == '#') continue;
			int tip = (i + j) % 2;
			for (int k = 0; k < 4; k++) {
				int nx = i + dx[k];
				int ny = j + dy[k];
				if (!ok(nx, ny)) continue;
				int sus = idx[nx][ny];
				v[tip][idx[i][j]].push_back(sus);
			}
		}
	}
}

bool find (int j, vector <vi>& g, vi& btoa, vi& vis) {
	if (btoa[j] == -1) return 1;
	vis[j] = 1; int di = btoa[j];
	for (int e : g[di]) {
		if (!vis[e] && find(e, g, btoa, vis)) {
			btoa[e] = di;
			return 1;
		}
	}
	return 0;
}

void dfsMatching (vector <vi>& g, vi& btoa) {
	vi vis;
	for (int i = 0; i < (int)g.size(); i++) {
		vis.assign((int)btoa.size(), 0);
		for (int j : g[i]) {
			if (find(j, g, btoa, vis)) {
				btoa[j] = i;
				break;
			}
		}
	}
	//return (int)btoa.size() - (int)count(btoa.begin(), btoa.end(), -1);
}

void dfs (int i, vector <vi>& g, vi& btoa, vi& vis) {
	if (vis[i]) return;
	vis[i] = 1;
	for (auto j : g[i]) {
		int sus = btoa[j];
		if (sus == -1) continue;
		dfs(sus, g, btoa, vis);
	}
}

int calc (int tip) {
	vi btoa(cnt[!tip], -1);
	dfsMatching(v[tip], btoa);

	vi atob(cnt[tip], -1);
	for (int i = 0; i < (int)btoa.size(); i++) {
		if (btoa[i] != -1) atob[btoa[i]] = i;
	}

	vi vis(cnt[tip], 0);
	for (int i = 0; i < cnt[tip]; i++) {
		if (atob[i] == -1) {
			dfs(i, v[tip], btoa, vis);
		}
	}
	
	int sol = 0;
	for (int i = 0; i < cnt[tip]; i++) {
		if (vis[i] == 1) sol++;
	}
	return sol;
}

int main () {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cin >> n >> m;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			cin >> c[i][j];
			if (c[i][j] == '#') {
				idx[i][j] = -1;
				continue;
			}
			int tip = (i + j) % 2;
			idx[i][j] = cnt[tip];
			cnt[tip]++;
		}
	}
	build_graph();
	cout << calc(0) + calc(1) << endl;
	return 0;
}